1. Field of the Invention
The present invention relates to an optical disk apparatus which detects a signal corresponding to a guide groove that is formed in a wobbling shape in a radial direction of an optical disk.
2. Description of the Related Art
In general, guide grooves indicative of track positions are preformed on recording-type optical disks such as a CD (Compact Disk), a DVD (Digital Versatile Disk), an HD-DVD (High Definition DVD) and a Blu-ray disk. The guide groove wobbles in a radial direction at predetermined cycles. The optical disk apparatus detects a signal (wobble signal) corresponding to the wobbling guide groove, and generates a timing clock at a time of data write on the basis of the detected signal.
Japanese Patent No. 3458502 discloses an optical disk apparatus which is capable of exactly detecting a wobble signal even in a case where a spot of light, which is reflected from an optical disk and is converged, deviates from the center of a photodetector due to an error in attachment position of the photodetector.
This optical disk apparatus includes a photodetector which has two light receiving units that are divided in a radial direction of the optical disk, and which detects the amount of light incident on each of the two light receiving units; an optical pickup head which converges a light beam on an information recording surface on the optical disk and converges reflective light from the optical disk onto the light receiving units of the photodetector; two auto-gain control (AGC) circuits which independently make constant the amplitudes of detection outputs which are obtained by receiving reflective light from a data-recorded area on the optical disk by the light receiving units of the photodetector; two fixed-gain adjustment circuits which independently adjust, with fixed gains, detection outputs, which are obtained by receiving reflective light from a data-non-recorded area on the optical disk by the light receiving units of the photodetector; and a selector which selects one of a subtraction output, which is obtained by performing subtraction between two outputs from the two AGC circuits, and a subtraction output, which is obtained by performing subtraction between two outputs from the two fixed-gain adjustment circuits. In this optical disk apparatus, an output from the selector is produced as a wobble signal.
In the meantime, in an optical disk apparatus, when data is to be recorded on a recording-type optical disk, a laser beam is radiated on the optical disk while the power of the laser beam is quickly varied in order to form pits on the optical disk. In accordance with the variation in power of the laser beam radiated on the optical disk, the amount of reflective light from the optical disk varies. As a result, the signal that is detected in accordance with the amount of reflective light also varies. Since the wobble signal also varies in accordance with the variation in amount of reflective light, the S/N ratio deteriorates.
In a conventional optical disk apparatus, in a case where a wobble signal is extracted from a signal that varies in accordance with the amount of reflective light, the variation in signal is reduced, for example, by using a band-pass filter. However, in the recording-type optical disk such as a DVD+R/RW disk, the frequency of the wobble signal that is used to control the recording position is increased in order to enhance the precision in position at the time of data recording. If the variation in the wobble signal having a high frequency is to be reduced by using a band-pass filter, it is possible that a signal that is read out in accordance with recorded data may be affected. Under the circumstances, there has been a demand for a technique which can improve the S/N ratio of a wobble signal, which varies in accordance with the variation in amount of reflective light from the optical disk, without using the band-pass filter, and thus can detect a stable wobble signal.